Stephen K. Bannon, President Trump's former chief strategist, was subpoenaed last week by the special counsel, Robert S. Mueller III, to testify before a grand jury as part of the investigation into possible links between Mr. Trump's associates and Russia, according to a person with direct knowledge of the matter. The move marked the first time Mueller is known to have used a grand jury subpoena to seek information from a member of Trump's inner circle. The special counsel's office has used subpoenas before to seek information on Trump's associates and their possible ties to Russia or other foreign governments.

The subpoena could be a negotiating tactic. Mueller is likely to allow Bannon to forgo the grand jury appearance if he agrees to instead be questioned by investigators in the less formal setting of the special counsel's offices in Washington, according to the person, who would not be named discussing the case.

But it was not clear why Mueller treated Bannon differently than the dozen administration officials who were interviewed in the final months of last year and were never served with a subpoena.

On Tuesday, Bannon testified behind closed doors before the House Intelligence Committee, which is also investigating Russia's meddling in the 2016 election and ties between the Trump campaign and Russia.

Assistant Attorney General Leslie R. Caldwell of the Justice Department's Criminal Division, U.S. Attorney Rod J. Rosenstein of the District of Maryland, Deputy Inspector General for Investigations John R. Hartman of the U.S. Department of Energy's Office of Inspector General (DOE-OIG) and Assistant Director in Charge Paul M. Abbate of the FBI's Washington Field Office made the announcement.